This invention relates to an amplifier circuit, in particular but not exclusively to an amplifier circuit for providing bandpass amplification at intermediate frequencies in radio receivers.
Amplifiers are widely used in the prior art for amplifying input signals applied thereto to provide amplified output signals. This is particularly important in radio receivers in which radiation received thereat generates corresponding antenna received signals which typically have an amplitude of microvolts. The radio receivers employ amplifiers therein to amplify such received signals to an amplitude in the order of millivolts to volts, for example to drive a loudspeaker. Since it is difficult to prevent amplifiers designed to amplify at radio frequencies from spontaneously oscillating, especially if they comprise cascaded gain providing stages, it is customary to heterodyne the received signals to lower intermediate frequencies whereat it is easier to provide a high degree of amplification and also provide more selective bandpass signal filtration.
In prior art radio receivers, it is therefore customary to provide a majority of signal amplification required at intermediate frequencies, namely frequencies lying intermediate between that of the radiation received and audio or video frequencies. For example, a radio receiver receives radiation at a frequency of 500 MHz and generates a corresponding antenna received signal also at 500 MHz. The receiver heterodynes the received signal to generate an intermediate frequency signal in a frequency range around 10.7 MHz which is then amplified and filtered, and finally demodulates the amplified intermediate frequency signal to generate a corresponding audio output signal having signal components in a frequency range of 100 Hz to 5 kHz.
Recently, because the radio frequency spectrum is becoming increasingly congested, there is a trend to use an ultra high frequency (UHF) range in contemporary communications systems, namely around 500 MHz; transmission at microwave frequencies, for example 1 GHz to 30 GHz is now also employed. Associated with this is a trend in modern radio receiver design to employ intermediate frequency amplification at several tens of MHz or greater; this is done in order to obtain adequate ghost image rejection associated with using heterodyne processes.
In modem mobile telephones, most signal amplification is provided in intermediate frequency amplifier circuits incorporated therein. These circuits comprise transmission amplifiers and associated surface acoustic wave (SAW) or ceramic filters to provide a narrow bandpass signal amplification characteristic; the circuits and their associated filters are conventionally referred to collectively as an xe2x80x9cintermediate frequency stripxe2x80x9d. Such transmission amplifiers consume significant power in operation, for example intermediate frequency amplifier circuits employed in mobile telephones typically consume between several hundred microamperes and several mA of current when operational.
In order to provide modern mobile telephones with extended operating time from their associated batteries, new types of battery have been researched and developed which provide enhanced charge storage to weight performance, for example rechargeable metal hydride and lithium batteries.
The inventor has appreciated, rather than concentrating on improving battery technology, that reduction in current consumption of intermediate frequency amplifier circuits in radio receivers is desirable to provide extended operating time from batteries. The invention has therefore been made in a endeavour to provide an alternative type of amplifier circuit, for example a circuit especially suitable for use at intermediate frequencies in radio receivers which is capable of requiring less power to operate.
It is known in the art, as described in a Japanese patent application JP 600127806A, to provide a microwave multistage amplifier comprising a cascaded series of inter-stage isolators connected to associated reflection amplifiers and delay circuits. The circulators are operable to hinder spontaneous oscillation from arising within the microwave amplifier.
According to the present invention, there is provided an amplifier circuit for receiving an input signal and providing a corresponding amplified output signal, the circuit characterised in that it comprises:
(a) a plurality of reflection amplifiers cascaded in series along a signal path and operative to amplify the input signal propagating in a forward direction therealong to provide the output signal; and
(b) connecting means for connecting the reflection amplifiers to form the signal path and for hindering signal propagation in a reverse direction therealong, thereby counteracting spontaneous oscillation from arising within the circuit, the connecting means incorporating switching means for alternatively switching a plurality of the reflection amplifiers such that each reflection amplifier is operable to receive signals propagating in the forward direction along the path and to output the signals after amplification to the signal path to propagate further in the forward direction therealong, the reflection amplifiers incorporating signal delaying means for delaying signal propagation to and from the reflection amplifiers.
The invention provides the advantage that incorporation of the delaying means enables the switching means to partition the input signal into discrete segments which are stored for a period of time within the delaying means, thereby providing the switching means with time to direct the segments in the forward direction along the signal path and prevent them from flowing in the reverse direction along the path which would give rise to spontaneous oscillation.
One skilled in the art would not expect it to be practicable to connect a plurality of reflection amplifiers together and obtain stable amplification therefrom because of spontaneous interfering oscillations which would arise during operation. The circuit addresses this problem by incorporating the connecting means which promotes intended signal amplification in the circuit and counteracts signal amplification giving rise to spontaneous oscillation therein.
Spontaneous oscillation is defined as self induced oscillation arising along a signal path providing amplification as a consequence of feedback occurring around or within the signal path.
Thus, conveniently, the delaying means comprises a plurality of delay lines, such that a delay line is interposed between each reflection amplifier and the switching means. This provides the advantage that each reflection amplifier can be isolated by the switching means for purposes of switching discrete signal segments.
Preferably, the delay lines are operative to provide bandpass signal transmission therethrough. This provides the advantage that the amplifier circuit is capable of providing a bandpass transmission characteristic appropriate for intermediate frequency amplifiers for use in, for example, radio receivers and mobile telephones.
In another aspect, the invention provides a method of amplifying an input signal and providing a corresponding amplified output signal, the method including the steps of:
(a) providing a plurality of reflection amplifiers cascaded in series along a signal path, and connecting means for connecting the reflection amplifiers to the signal path, the connecting means operative to promote signal propagation in a forward direction along the path and counteract signal propagation in a reverse direction therealong, the connecting means incorporating switching means for alternatively switching a plurality of the reflection amplifiers such that each amplifier is operable to receive signals propagating in the forward direction along the signal path and to output the signals after amplification to the signal path to propagate further in the forward direction therealong, the reflection amplifiers incorporating signal delaying means for delaying signal propagation to and from the reflection amplifiers;
(b) receiving the input signal at the signal path;
(c) directing the input signal through the connecting means to one of the reflection amplifiers for amplification therein to provide an amplified signal;
(d) directing the amplified signal in the forward direction to another of the reflection amplifiers for further amplification therein;
(e) repeating step (d) until the amplified signal reaches an output of the signal path; and
(f) outputting the amplified signal as the output signal from the signal path.
The method provides the advantage that, during amplification, the signal is selectively directed from amplifier to amplifier in a forward direction along the signal path, thereby counteracting any of the amplifiers reamplifying the input signal and hence preventing any feedback loops being established in which spontaneous oscillation can arise.